This is the most useful language in the world, no questions asked. Anyone having anything to do with other countries, be it in science, tourism, trade, finance or culture, will be extremely well rewarded if he masters the language decently. Most books are published in English, and most are translated in English. The best selling movies are American, and translation is a pain-in-the-ear. English, even bad, is the standard language of scientific publications, aviation, tourism, finance, and diplomacy. Anyway, if you're reading this, you are already sold to the language, so why insist?
Although this is not the primary attraction of this language, it is easy to fall in love with its colorful expressions, alliterative riddles, easygoing approach toward word creation or importation. My personal reference for prose is The Economist, a conservative British weekly with main circulation in the USA.
Almost everybody claims to speak English, so I wonder where people can find any chic at all in this language. And yet. Some Germans are especially disconcerting when they show off their English and look back at their fellow Germans with a satisfied grin.
Total number of speakers is more than 600mio, but this figure must be qualified by the fact that most people you will meet when traveling or meeting people from other countries will speak English. Early in the next century, more people will speak English as a second language than those who learned it from their mother.
USA, Canada, England, Australia, New Zealand, Ireland, Scotland, Wales, Belize and some other ex-British colonies.
The reference accent seems to be that of the American TV and movies, for the better or worse. The heaviest variations from this "standard English" occur, as always, in backward areas. Rural USA is already something but try Northern England, especially among lower class people to get a real cultural lag. Accent notwhistanding, there are some funny differences like flat/apartment or lift/elevator that you can master in one afternoon if you find the right list.
Unless you have a very specific area in mind, this is THE traveler's language, earning you at list understanding if not sympathy almost everywhere in the world.
Excellent movies, pitiful TV, a lots of excellent literature, both contemporary and older, and the best thing-books you can find. On almost any topic the best books are available in English, even if they have been translated. And they are cheaper too. Pop music has its fans and some singers deserve your understanding their lyrics.
English uses some difficult sounds, like those in thing or soothe. These sounds are, in my opinion, impossible to master perfectly unless you have lived in an English-speaking environment before you turned 16-18.
Pretty logical and not very difficult to master. Adjectives do not vary, conjugation is almost Esperanto-like, but orthograph is tricky. The problem is that with such a weak verbal system and such easy rules to turn adjectives into verbs, the beginner may be puzzled by some phrases. Another difficulty is that of the verbs that use a separate particle like to set off or to pop out. I advise the learner to review lists of common verbs to save time, because they are especially tricky to look in the dictionary.
The English language has one of the most extensive vocabulary around, with more than than 460'000 different words in the Webster Dictionary alone. This comes from the French/German origin, and means that you often have at least two words with almost exactly the same meaning. However, most of the dictionary only lives in literature and scientific publications.
I rate this language as **, that is, quite easy to learn, because of its ubiquity, and grammar. Almost everyone will tolerate a bad English if they still understand you, and anyway they will have heard so much of it by the time you talk to them that they won't mind. In most parts of the world, you only have to go to the cinema, open the TV or buy newspapers to have the language imposed on you so there's really no excuse for not speaking it at least decently.
The big question, I think that two years of moderate study should be sufficient for all but the most lazy people, and you can definitely learn it in 6 months. As to the number of hours, I think that 250 hours is a safe estimation.